Bright alloy plating



Patented Feb. 17, 1948 umrap STATES PATENT OFFICE" I amen BRIGHT ALLOY PLATING George w. rel-meat. Belleville, a. 1., anignor to Westinghouse Electric Corporation. East Pittsburgh, 2a., aeorporatien of Pennsylvania This invention'relates to electroplating and, more particularly, to electroplating of bright ternary alloy coatings upon metal surfaces to provide for appearance and corrosion-resistance.

This application is a continuation-in-part of my copending patent application Serial No. 468,253, filed December 8, 1942, and entitled Bright alloy plating," which has become aban- 'doned.

In applying electroplated coatings to metal surfaces in order to provide for attractive appearance and predetermined color and corrosionresistance, only a limited number of metals are ordinarily available. At the present time. certain of the more usual electroplating metals are scarce, and the number of electroplating metals available to the industry has been sharply reduced. It is quite important to employ metals which are not so scarce for plating purposes, and a feature of this invention is to provide desirable plating by using metals obtainable in relative abundance.

In the copending patent application of George Jernstedt, Serial No. 458 399, filed September 15, 1942, entitled Bright alloy plating" which has become abandoned, is disclosed the electroplating of bright silvery coatings of metal comprising a ternary alloy of copper, tin and zinc. The ternaryalloy is electroplated from anodes of a predetermined composition. A highly satisfactoryI silvery electroplate is produced by following the practice set forth therein. v

According to the present invention, the same ternary alloy is electroplated from anodes of the predetermined composition disclosed in my copending patent application with improved results by the application of certain addition agents to the electrolyte. In particular, it has been discoveredthat certain specific betaine brightening agents and anti-pitting agents applied thereto result in an electroplate of exceptional corrosion resistance and a high degree of brightness.

The object of this invention is to provide for an improved method of electroplating a ternary alloy as a silvery deposit of optimum brightness and without pitting,

A further object of this invention is to introduce into alloy plating baths addition agents which function to prevent pitting and act as brighteners during plating.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

A fuller understanding of the nature and objects of this invention will be had from the following detailed description.

The ternary alloy which is electroplated in practicing the present invention may consist of from to 15% copper, 15% to 30% tin, and 5% to 20% zinc. In some cases, these proportlons may be departed from to a slight extent.

No Drawint. Application mm" 21,1945. Serial He. seems comm. (cl. 204-44) The presence of minor amounts of other metals or impurities is believed not to aifect the nature of the electroplated product. Particularly good results are obtained if the copper content of the electroplate is from to 65%, the tin from 20% to 30% and the zinc from 8% to 20%. The

tin content of the anodes may be two or three.

percent more than the plated alloy. since during electroplating operations a small fraction of the tin precipitates out of the electrolyte.

In making the anodes, it has been found that the ternary alloy is conveniently prepared by melting the required weight of copper in a crucible with the predetermined amount of tin and zinc added just prior to the casting of the melt into suitable anodes. A brass of suitable composition may be melted initially and tin added in producing the ternary alloy. Permanent molds or, in some cases, sand molds may be used in casting anodes. During the casting operation, supporting hooks of a conducting metal such as steel may be cast into the anodes to provide for supporting them from the anode conductor bar in the electroplating apparatus. In case a conductor anode frame or holder is to be employed. the ternary alloy may be cast into any desired shape, such as balls, suitable for putting into the anode frame.

The anodes prepared in this manner or by the. other means, are suspended in an electrolyte having the following constituents within the limits indicated:

Sodium hydroxide, about 0.25 to 0.8 ounce per gallon to give a pH of from about 12 to 13.

When the composition is maintained within the limits indicated. plating may be accomplished with satisfactory results.

An electrolyte that has been satisfactory for both barrel and still-tank plating has the following composition:

, Oz. per gal. Copper cyanide 0.5

Zinc cyanide 0.27 Sodium sta 0.22 Sodium cya 3.75 Sodium car 4.0

Sodium hydroxide as The pH of this electrolyte, when maintained in the range of from about 12.6 to 13 produces. Analysis of the ternary.

excellent electroplate. alloy electroplate produced by this last electrolyte shows copper varying from 55% to Zinc may be added as zinc sulfate. Other zinc.

tin and copper salts, which are soluble in a cyanide solution may be employed in preparing the electrolyte. It will be understood that potassium salts may be used instead of sodium salts, allowance being made for the diilerence in molecular weight. The electrolyte is prepared by dissolving the iinely powdered salts in the predetermined quantity of water.

The electroplating of the ternary alloy in the electrolyte of the above composition may be carried out with greatly improved results if addition agents are added to the electrolyte. It has been discovered in particular that the electrodeposlt is rendered more continuous, more uniform, and brighter whereby a more corrosion resistant coating is produced with less pitting if certain betaine compounds are added. Betaines generally include the N+trialkyl derivatives of amino acids. For the purpose of this invention, betaines which have given the best results are those which have at least one non-cyclic hydrocarbon radical containing from 10 to carbon atoms. For example, trimethyl-C-decyl alpha betain'e is a suitable addition agent. It has the following accepted formula:

cnor-N-cn-c=o The betaines which may be employed in the practice of my invention are quite numerous and the following examples are illustrative of the general class..

Trimethyl-C-decyl alpha betaine Trimethyl-C-cetyl alpha betaine Trimethyl-C-cetyl gamma betaine Dillzft yl, beta-hydroxyethyl-C-cetyl alpha bet ne Methyl, di(beta-hydroxyethyl) C cetyl alpha betaine Trimethyl-C-lauryl alpha betaine Trimethyl-C-lauryl gamma betaine Trimethyl-C-stearyl alpha betaine Dimethyl-N-stenyl alpha betaine Dimethyl-N-stenyl-C-methyl alpha betaine Methyi-N-distenyl alpha betaine Dimethyl-N-stenyl-C-cetyl beta betaine Dimethyl-N-decyl, beta-hydroxy gamma betaine Dimethyl-N-heptadecyl alpha betaine Dimethyl-N-octadecyl-C-methyl alpha betaine Dimethyl-N-octadecyl beta betaine Dimethyl-N-octadecyl, beta-hydroxy gamma betaine The betaine compounds having from 15 to 18 carbon atoms have givenroutstanding results when added to the ternary alloy electrolyte.

The Proportion of betaine which has produced the most satisfactory results is from 0.01 to 1 ounce per gallon of the electrolyte. From time to time, small amounts of the betaine should be added in order to replace drag-out losses.

In some cases, in addition to the betaines, sodium disulfate, tri-sodium phosphate and sodium sulfate have been added with benefit in producing a bright electroplated coating. In general, the betalnes are fully compatible and stable with the various other addition agents and the constituents of the plating bath. The bath or electrolyte is operated at a pH of from between 1 l and 13.

In electroplating an alloy composed of 55%- 4 for example, to produce a white silvery plate, the voltage between the anode and cathode should bemaintainedwithinthelimitsof8tobvolts where the anodes and cathodes are separated at standard plating distances. A higher voltage will be required if the anodes and cathodes are separated by more than the standard distance. The current density is between 15 and 20 amperes Per Square root under these conditions.

The apparatus may be operated at a temperature of 140 F. to 160 F. over a wide range of amperes per unit area. It is possible to obtain coatings of 0.0001 to 0.0005 inch thick in a short time. For most purposes, coatings of this thickness have adequate covering power and corrosion-resistance. In some cases. where it is desirable to have a thicker coating, plates varying from 0.001 to 0.002 inch thick and even heavier and which are quite bright have been obtained by electroplating for greater periods of time.

The electroplate alloy is surpassed only by silver in reflectivity. Tests indicate that its reflectivity is approximately 85% of that of freshly plated silver. However, an advantage over silver plate is that the ternary alloy plate deteriorates in reflectivity very slowly, whereas silver becomes tarnished rapidly under normal circumstances.

The corrosion-resistance of the ternary alloy electroplate is superior to nickel, tin, chromium, and silver when these latter metals are plated on copper or copper alloys. For example, 0.0005 inch thickness of nickel on copper when subjected to salt-spray tests will fail in less than 100 hours. The ternary alloy 0.0002 inch thick plated on copper regularly passes a 200 hour saltspray test without any visible corrosion. Very few electroplated metals have the corrosionresistance that the ternary alloy of this inven- 0 tion has.

An addition advantage of the ternary allay applied to copper, in particular, is that the alloy can be soldered to as effectively and easily as to tinned parts. This feature is exceedingly important in the fabrication of instruments, meters. and other electrical apparatus, for example, where contacts and other parts must be soldered in assembling the apparatus. It is essentially non-magnetic and therefore is useful in instrument manufacture.

The alloy may be plated upon many metals used as a base. be directly plated; though, to secure maximum corrosion resistance, steel is initially copper or brass plated and then ternary alloy plated. Copper, brass and other copper alloys are excellent 7 bases for the alloy plate.

Hardness tests indicate that the ternary alloy electroplate is much harder and more resistant to abrasion than electroplated coatings of nickel, tin orv silver.

A particular advantage of the alloy plating bath described herein over the usual single metal plating baths is due to the greatly superior throwing power of the ternary alloy. The alloy has a greater throwing power than nickel, tin, silver, or copper. For this reason, the ternary alloy coatings need only be about two-fifths as thick as a nickel, tin, or silver plate to give as good coverage and corrosion-resistance to metal surfaces.

Since certain changes in carrying out the above processes and certain modifications in the compositions which embody the invention may be copper, 20%-80% tin, and 8%-20% zinc, 1s made without departing from its scope, it is in- Steel and ferrous metals may tended that all the matter contained in the above description or shown in the accompanying drawin-g shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention: 1. An electroplating bath for depositing a bright silvery alloy plate composed of 50% to 75%- copper, 15% to 30% tin and 5% to 20% zinc comprising, in combination, an aqueous electrolyte composed offrom about 0.5 to 5 oz. of tree cyanide per gallon, 0.2to 0.5 oz. of copper per gallon, 0.05 to 0.2 oz. of tin Per gallon, 0.1 to 0.5 oz. of zinc per gallon, alkali metal hydroxide and 2.0 to 12.0 oz. per gallon of an alkali metal carbonate. the electrolyte being maintainedat a tile of this patent:

lyte composed of from about 0.5 to 5 oz. of free cyanide per gallon, 0.2 to 0.5 oz. of copper per gallon, 0.05 to 0.2 oz. of tin p r gallon, 0.1 to 0.5 oz. of zinc per gallon, alkali metal hydroxide and 2.0 to 12.0 oz. per gallon of an alkali metal carbonate, the electrolyte being maintained at a pH value of from about 11 to 13, and an antipitting and brightening agent composed of from 0.01 to 1.0 oz. per gallon of a betaine having at least one non-cycle hydrocarbon radical which contains 15 to 18 carbon atoms.

GEORGE W. JERNSTEUP.

REFERENCES CITED The following references are of record in the UNITED STATES PATENTS 

